The invention relates to pressurised steam boilers and their control, to a method and apparatus for detecting the level of water in a steam boiler and to a method and apparatus for assessing the mass flow of steam from a steam boiler.
In a known arrangement of a pressurised steam boiler, water is fed into the boiler at a controlled rate and is heated in the boiler to convert the water to steam. The heat required to convert the water to steam is provided by a burner whose hot products of combustion are passed through ducts in the boiler and then exhausted. The steam boiler is controlled by a boiler control system, which receives information from sensors indicating inter alia the level of water in the boiler and the presence of steam in the boiler, and which controls the flow rate of water into the boiler as well as sending a control signal to a burner control system that controls the burner The burner control system controls inter alia the flow of fuel and gas to the burner head in dependence upon a demand signal received from the boiler.
Pressurised steam boilers are potentially very hazardous because of the very high pressure that is maintained in the boiler and it is therefore essential for such boilers to have control systems that are extremely safe. One factor that is taken into account to ensure the safety of a system is the importance of maintaining the water level in the boiler within predetermined limits. The internationally recognised safety regime concerning adequate water level in pressurised steam boilers requires sensing arrangements to detect a first low water level (xe2x80x9cfirst lowxe2x80x9d) below the normal operating range of the boiler and also to detect a second low water level that is even lower than the first low water level. When the first low water level is detected, the boiler control system sends a signal to the burner control system causing the burner to be switched off. Provided the water level then rises back above the first low water level the boiler control system sends a further signal to the burner control system allowing the burner to restart. If, however, the water level continues to fall and reaches the second low water level, the boiler control system sends a further signal to the burner control system preventing it from restarting without manual intervention. The requirement for manual intervention is inconvenient, but is regarded as a necessary safety requirement.
The false triggering of either the first low or second low is costly. The effect of a false triggering at the first low is to turn off the burner; at best that may simply lead to less efficiency because the burner is switched completely off rather than simply being turned down to a lower firing rate; in a worst case, however, as will be explained below, the false triggering bay lead to the burner being switched off at a time when the demand for heat in the boiler is especially high. False triggering at the second low is more damaging because it is likely to last longer given that the burner can be restarted only after manual intervention.
False triggering can occur without any fault in the equipment. In particular, it is not unusual for there to be a sudden demand for steam from a steam boiler; in that case there may be a significant drop in pressure within the boiler which can cause the water level in the boiler to rise (because of the small bubbles of compressed gas trapped within the water in the boiler). The reduction in pressure rightly leads to a signal passing from the boiler control system to the burner control system to increase the firing rate of the burner, while the increase in water level in the boiler causes the usual water flow into the boiler to be reduced or stopped. As the system then recovers and the pressure in the boiler rises, the water level in the boiler falls quickly and may well fall below the xe2x80x9cfirst lowxe2x80x9d leading to the burner being turned off at a time when it should be operating, probably at full capacity. It is even possible that the fall in water level will reach the xe2x80x9csecond lowxe2x80x9d so that the burner remains off until an operator resets the system.
Safety considerations also have an impact on the techniques that are employed to measure the level of water in the boiler. Because of the importance of detecting the xe2x80x9cfirst lowxe2x80x9d and the xe2x80x9csecond lowxe2x80x9d, separate probes are used to detect each of the levels; whilst one capacitative probe may sometimes be provided to sense water levels within the normal operating range, respective conductive probes, which sense whether or not they are in the water, but give no further indication of water level, are provided to detect the xe2x80x9cfirst lowxe2x80x9d and the xe2x80x9csecond lowxe2x80x9d. Often other conductive probes are set at other levels so that those other levels can be detected in a similar way. Thus a large number of separate probes are provided. A capacitative probe is not regarded as sufficiently reliable for detecting the xe2x80x9cfirst lowxe2x80x9d and the xe2x80x9csecond lowxe2x80x9d water levels. One particular concern is that the signals for such probes may be affected by stray electromagnetic radiation generated by devices in the vicinity of the probes.
Operators of pressurised steam boilers frequently purchase steam flow meters to measure the steam flows in the steam exit lines from each of the boilers. A frequent reason for installing such meters is for auditing purposes, to enable the amount of steam exported from the boiler to be compared to the amount of fuel used by the boiler. Such meters are, however, expensive.
It is an object of the invention to provide an improved method and apparatus for controlling the operation of a steam boiler.
It is a further object of the invention to provide a method and apparatus for controlling the operation of a steam boiler in which the likelihood of a burner being shut down unnecessarily is reduced.
It is a further object of the invention to provide an improved method and apparatus for detecting the level of water in a pressurised steam boiler, and especially to provide a method and apparatus in which the number of probes that are required is reduced.
It is a still further object of the invention to provide a method and apparatus for assessing the mass flow of steam from a pressurised steam boiler without resorting to a steam flow meter.
According to the invention there is provided a method of controlling the operation of a steam boiler heated by a burner, the method including the following steps:
a) monitoring the level of water in the boiler,
b) monitoring the pressure of steam in the boiler,
c) monitoring the firing rate of the burner, and
d) controlling the flow rate of water into the boiler having regard to the signals resulting from a) and
b) and, at least for some signal conditions, also having regard to signals resulting from c).
By using the firing rate of the burner as one of the control inputs for determining the flow rate of water into the boiler and in that respect combining the burner control system and the boiler control system, it becomes possible to effect a more appropriate control of the water, reduce the number of times that the water level in the boiler falls below a first low water level at which the burner is switched off and thereby improve the efficiency of the boiler.
Whilst it is within the scope of the invention for the control of the flow rate of water into the boiler always to take account of signals resulting from monitoring the firing rate of the burner, it may be that the signals resulting from monitoring the firing rate of the burner are taken into account in a limited set of circumstances only. It is for example preferred that when
i) the monitoring of the level of water in the boiler shows a rate of increase above a predetermined level,
ii) the monitoring of the pressure of steam in the boiler shows a reduction in pressure at a rate above a predetermined level, and
iii) the monitoring of the firing rate of the burner shows that the firing rate is increasing at a rate above a predetermined level,
the controlling of the flow rate of water into the boiler is such that it does not necessarily reduce the rate of flow into the boiler.
Preferably, said controlling of the flow rate of water into the boiler is such that it does not reduce the rate of flow into the boiler, unless the level of water in the boiler is above an upper normal working limit. In a case where there is a sudden demand for steam so that the steam pressure drops quickly and the water level in the boiler increases rapidly, the flow rate of water into the boiler is controlled in dependence upon what is concurrently happening to the firing rate of the burner: if the firing rate of the burner is increasing at a rate above a predetermined level, then that is an indication that the drop in steam pressure is a result of increased demand and that the increase in boiler water level is misleading, and the rate of flow of water into the boiler is not reduced. Since water continues to flow into the boiler the likelihood of the water level dropping below the first or second low water levels is significantly reduced.
An example of a situation where the monitoring of the firing rate would still lead to a reduction in the rate of flow of water into the boiler is given below: when
i) the monitoring of the level of water in the boiler shows an increase in level but at a rate of increase below a predetermined level.
ii) the monitoring of the pressure in the boiler shows an increase in pressure but at a rate of increase below a predetermined level, and
iii) the monitoring of the firing rate of the burner shows that the firing rate is reducing
the controlling of the flow rate of water into the boiler is such that it does reduce the rate of flow into the boiler.
Preferably, input and output signals relating to all the monitoring and controlling steps are passed into or transmitted from a common control unit that also controls the operation of the burner. The integration of the boiler control unit and burner control unit into a single control unit simplifies, improves and makes cheaper the control of the burner and boiler.
Where reference is made above to a rate of increase above a predetermined level, it is within the scope of the invention for the rate of increase to be at any level above zero. It is preferred, however, that the predetermined level corresponds to what is to be regarded as a normal rate of increase during ordinary operation of the burner and boiler. Appropriate predetermined levels may be determined by a commissioning engineer during commissioning of the system and a rate of increase may be obtained by measuring the increase in values over a time period of the order of 20 seconds.
Where reference is made to monitoring a variable, it should be understood that the variable itself may not be directly sensed but rather one or more other variables, from which the variable being monitored can be calculated, may be sensed. For example, the firing rate of the burner need not be directly sensed and the pressure of the water in the boiler may be sensed to indicate the pressure of the steam.
In an especially preferred method, the step of monitoring the level of water in the boiler includes the steps of providing a pair of capacitance probe assemblies mounted in the boiler with each of the probes extending through a range of water levels, the probes being arranged such that the capacitance of each probe varies according to the level of the water, and of measuring the capacitance of each probe, comparing the capacitances to one another to check that they match and using the measurement of the capacitance as an indication of the water level. By providing a capacitance probe assembly to measure the water level in the boiler it becomes possible to measure a wide range of levels and, if desired, all the intermediate levels without a large number of probes. Furthermore, by providing a pair of probes that measure the same levels, safety can be considerably improved. Of course, more than two probes can be employed, if desired.
The range of water levels through which the probes extend preferably includes a first low water level below the normal working range. Thus the probes are preferably used to detect the xe2x80x9cfirst lowxe2x80x9d Furthermore, the range of water levels through which the probes extend preferably includes a second low water level below the first low water level. Thus the probes are preferably also used to detect the xe2x80x9csecond lowxe2x80x9d. Conventional capacitative probes have not been regarded as satisfactory for detecting the xe2x80x9cfirst lowxe2x80x9d and xe2x80x9csecond lowxe2x80x9d because of the importance, from a safety point of view, of that detection. We have found, however. that by using a pair of probes to make the same measurements it is possible to provide a very safe detecting arrangement.
It is still further preferred that the range of water levels through which the probes extend include all other water levels that are to be detected. In that case there is no need to provide any other water level detectors apart from the probes. The further water levels detected by the probes may be the limits of the normal working range of water level and/or a high water level above the normal working range.
Each of the capacitance probes preferably projects downwardly from an upper region of the boiler housing. Each probe preferably comprises an elongate core of electrically conducting material surrounded by a sleeve of electrically insulating material.
Preferably the pair of capacitance probe assemblies are substantially identical.
Each capacitance probe assembly preferably includes in addition a reference capacitance whose capacitance value is sensed alternately with the probe capacitance value. By providing such a reference capacitance value in each probe assembly, it is possible to detect any distortion of the sensed value of capacitance that might arise from, for example, electromagnetic radiation. Any such distortion in the sensed value of the reference capacitance may be used to adjust the sensed capacitance value of the capacitance of the probe and/or may be used to switch off the burner as a safety precaution.
Preferably the measurement of the capacitance of one probe alternates with the measurement of the capacitance of the other probe.
An especially preferred method of the invention further includes the step of assessing in a control unit the mass flow of steam from the boiler by processing of input signals including ones enabling assessments to be made of:
a) the heat generated by combustion in the burner
b) the temperature and pressure of the steam generated by the boiler
c) the heat dissipated other than in the steam.
It should be understood that a designer is able to make some selections as to how accurate the assessments of a) to c) above are to be and therefore how many variables are to be measured and how accurately they are to be measured. For example, in order to assess the heat dissipated other than in steam an operator might merely measure the temperature of the combustion products and assume a certain further dissipation of heat by other means such as conduction, convection and radiation from the boiler housing
By making an assessment of the mass flow of steam from measurements of other variables, the need for an expensive steam flow meter is avoided. Although it may appear that the measurement of several other variables in order to assess the steam flow is unnecessarily expensive and complicated, that need not be so because the other variables may be mainly or entirely ones that are being measured anyway for the purpose of controlling the operation of the pressurised steam boiler and burner.
Variables measured to assess the heat generated by combustion in the burner may include the rate of feeding of s fuel to the burner, and/or the composition of the combustion products.
Variables measured to assess the heat dissipated other than in the steam may include the temperature of the combustion products and/or the rate of feeding fuel to the burner.
In GB 2169726A, the description of which is incorporated herein by reference, a fuel burner control system is described which includes flue gas sampling and analysing apparatus and which also includes a burner controller which is the subject of GS 2138610A, the description of which is also incorporated herein by reference. That control system already receives inputs relating to the rate of feeding fuel to the burner, the composition of the exhaust gases and the temperature of the exhaust gases. Furthermore it is common for a pressurised steam boiler control system to include sensors for measuring the temperature and pressure of the steam generated by the boiler. Thus it can be seen that all the variables required for the assessment of the mass flow of steam from the boiler may already be available without any extra sensors being required. If desired, however, one or more extra sensors may be provided. For example, a sensor for measuring the temperature of the water being fed into the boiler may be provided.
The assessment of the mass flow of steam from the boiler may be used only as a measure of the flow at a moment in time, or it may also or alternatively be used to provide an assessment of the aggregate amount of steam generated over a certain extended period of time. In the latter case, it may be necessary to allow for other losses within the system, when making the assessment, for example it may be appropriate to assume that a certain percentage of heat is lost during blow down of a boiler. For example an overall loss of 6 percent might be allowed for.
The present invention further provides a method of monitoring the level of water in a pressurised steam boiler, the method including the steps of providing a pair of capacitance probe assemblies mounted in the boiler with each of the probes extending through a range of water levels, the probes being arranged such that the capacitance of each probe varies according to the level of the water, and of measuring the capacitance of each probe, comparing the capacitances to one another to check that they match and using the measurement of the capacitance as an indication of the water level.
The present invention yet further provides a method of assessing in a control unit the mass flow of steam from a pressurised steam boiler by processing input signals including ones enabling assessments to be made of:
a) the heat generated by combustion in the burner
b) the temperature and pressure of the steam generated by the boiler
c) the heat dissipated other than in the steam.
Although the invention has been defined above with reference to a method, it will be understood that it may also be embodied in an apparatus comprising a pressurised steam boiler. Thus the present invention still further provides a pressurised steam boiler including
a boiler housing for containing water in the boiler,
a burner for heating water in the boiler and converting the water into steam,
a water level detector for monitoring the level of water in the boiler,
a pressure detector for detecting the pressure of steam in the boiler,
a firing rate detector for detecting the firing rate of the burner, and
a control unit which receives input signals from the water level detector, the pressure detector and the firing rate detector and is operative to control the flow rate of water into the boiler in dependence upon said input signals.
The present invention still further provides a pressurised steam boiler including:
a boiler housing for containing water in the boiler, and
a water level detector for monitoring the level of water in the boiler, the water level detector comprising a pair of capacitance probe assemblies mounted in the boiler housing with each of the probes extending through a range of water levels, the probes being arranged such that the capacitance of each probe varies according to the level of water, and a control and processing system for measuring the capacitance of each probe, comparing the capacitances and providing an output signal indicative of water level based on the capacitance measurements.
The present invention still further provides a pressurised steam boiler including:
a boiler housing for containing water in the boiler,
a burner for heating water in the boiler and converting the water into steam,
a pressure detector for detecting the pressure of steam in the boiler,
a temperature detector for detecting the temperature of steam in the boiler,
a fuel flow detector for measuring the flow rate of fuel into the burner,
a further temperature detector for detecting the temperature of the exhaust gases,
a control unit for receiving and processing input signals from all of said detectors and for assessing indirectly the mass flow of steam from the boiler.